Process of forming a red, cuprous oxide coating on copper



States ABSTRACT OF THE DISCLOSURE The method of forming a red, cuprousoxide coating on copper which comprises heating a copper base alloy inan inert atmosphere at a temperature of from 1100 to 1900 F. and coolingsaid heated alloy to a minimum of 200 F. in an oxygen deficient zone.

The present invention relates to processes for the formation of a redcoating on copper.

It is highly desirable, for architectural, decorative and other purposesto provide copper or copper base alloy surfaces which have a durable redappearance.

Generally, copper surfaces when exposed to indoor atmospheres willtarnish and present an undesirable surface appearance. When exposed tooutdoor weathering copper surfaces become discolored and unsightly.Heating copper to elevated temperature in air results in a formation ofblack oxide coating which has no adherence and readily flakes off.

When an esthetic appearance is required in outdoor applications ofcopper, the formation of a green patina is relied upon. This takes manyyears to form and untilit does the copper surface is generallydiscolored and unsightly. For indoor applications where esthetic appealis required, the copper must be either polished frequently or coatedwith a clear lacquer. Frequent polishing is obviously inconvenient, timeconsuming and expensive, Whereas the durability of the lacquer coatingis relatively limited and thus cleaning and recoating is required atfrequent intervals.

Accordingly, it is highly desirable to devise a method for coloringcopper or copper alloy surfaces to provide a durable, estheticallypleasing, red appearance which overcomes the above disadvantages, isimmediately available after the practice of the invention and is stableunder conditions of outdoor and indoor exposures.

Accordingly, it is an object of the present invention to provide aprocess for forming a red oxide coating on copper.

It is a further object of the present invention to provide a red coatingas aforesaid which is highly decorative and pleasing and ischaracterized by the retention of its decorative and pleasing appearanceover a long period of time under a wide variety of conditions.

It is a further object of the present invention to provide a process asabove which is inexpensive and readily usable on a commercial scale.

'It is an additional object of the present invention to devise a processwhich provides a durable, esthetically pleasing, colored article asabove with the durable color being available immediately after practiceof the invention and being retentive under a wide variety of outdoor andindoor conditions.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the present invention it has now been found that theforegoing objects may be readily obtained. The process of the presentinvention forms a durable red cuprous oxide coating on copper by:heating a copper base alloy in an inert atmosphere at a temperature offrom 1100 to 1900 F. and cooling in an oxygen- 3,398,Z8 Patented Aug.20, 1968 deficient zone, preferably oxygen-free or substantiallyoxygen-free. The cooling should be done in an oxygendeficient zonecontaining less than 0 .0001% oxygen. Naturally, the greater the amountof oxygen in the oxygendeficient zone, the more rapid ,the cooling mustbe. In the event very small amounts of oxygen are present in theoxygen-deficient zone, the cooling may be more slowly. In general, it ispreferred for practical considerations that the cooling should be asquickly as possible, i.e., cool to ambient temperature in from 10seconds to three minutes.

In the preferred embodiment, after the heating step the following stepsare performed: exposing said heated alloy to oxygen, preferably air, forfrom 3 to 20 seconds; and cooling said alloy in an inert atmosphere.

It has been found that performance of the foregoing process results in acopper surface having a compact, strongly adherent cuprous oxide coatingthereon which has a pleasing red appearance. Furthermore, it has beenfound that the surface color or appearance is not noticeably aifected byseveral years indoor exposure, and outdoor exposure results in asoftening and mellowing of the color.

By appropriate masking techniques, it is possible to produce designs onthe copper alloy surface, such as lettering, numbering or estheticcharacterizations.

It has been surprisingly found in accordance with the present inventionthat the process conditions of the present invention are critical inobtaining the desired results.

In general, any copper base alloy may be used, i.e., any alloycontaining a major proportion of copper. Since the process of thepresent invention depends upon the formation of cuprous oxide, coppermust be the main oxide-forming constituent in the alloy which is used.Typical alloys which may be employed include, but are not limited to,the following: high purity copper, tough pitch copper, and OFHC copper(oxygen free high conductivity). Typical alloying su-bstituents includebut are not limited to the following: silver; gold; platinum; palladiumup to 5%; tin up to 2%; nickel up to 12%; zirconium up to 20%; and zincup to 10%.

Similarly, the particular form of the copper alloy is not especiallycritical but is governed by the desired application. For example, copperfoil, copper plate, or copper sheet may be readily employed.

The chosen copper base alloy is heated in an inert atmosphere at atemperature of from 1100 to 1900 F. When heated at temperatures under1100 F, the alloy does not attain the desirable red color of the presentinvention, but rather achieves a light orange to almost brasslikeappearance. It is critical that the alloy be heated in an inertatmosphere. The inert atmosphere heating zone should be substantiallyoxygen-free since oxygen present during the heating step develops ablack cupric oxide coating on the copper.

Typical inert atmospheres wihc-h may be provided are argon, nitrogen,helium, carbon dioxide, carbon monoxide and steam.

The length of time of maintaining the alloy in the temperature range1100 to 1900 F. is not particularly critical. The alloy may bemaintained in the temperature range for longer or shorter times asdesired, with the longer times naturally providing the thicker coatings.In general, the alloy is held in the temperature range for at least 3minutes.

The particular temperature chosen is governed by the particular shade ofred desired and the particular alloy used. In general, the higher thetemperature, the darker are the colors.

The alloys should be cooled to a minimum of 200 F. in theoxygen-deficient zone, and preferably the alloy is cooled to ambienttemperature in the oxygen-deficient zone.

If the copper is then cooled in an oxygen-deficient zone, it may becooled in the heating zone with the desired amount of oxygen or airmetered in and cooled at the chosen cooling rate. Alternatively, thecopper may be transferred to a separate zone.

In the preferred embodiment, after the alloy is heated at the desiredtemperature, the alloy is then exposed to oxygen and preferably air orair containing oxygen for from 3 to 20 seconds. If the heated alloy isexposed in excess of 20 seconds, a dark or blackened coating isattained. After the exposure for the desired period of time, the alloyis then cooled in an inert atmosphere. The alloy should be cooled in theinert atmosphere to a minimum of 200 F, preferably in less than threeminutes. Naturally, if desired, the alloy may be cooled in the inertatmosphere to room temperature.

The principle of the present invention is the heating of high coppercontent metal in an oxygen-free or inert atmosphere and then limitedexposure to air or oxygen so that cuprous oxide coating is formed ratherthan a cupric oxide coating. The highly protective nature of the cuprousoxide is due to the fact that when it forms on the surface, it is undercompressive stress resulting in a strongly adherent continuous coating.

Naturally, the present process may be performed either in batch orcontinuous manner. For example, strip may be preheated in an inertatmosphere, passed through an air or oxygen atmosphere for limitedoxygen exposure and then passed through a cooling zone deficient inoxygen.

The present invention and the improvements resulting therefrom will bemore readily apparent from a consideration of the following illustrativeexamples.

EXAMPLE I A coupon of tough pitch copper measuring 2%" x /2" x .022" wascleaned with steel wool until it was bright in appearance. It was thenplaced in a stainless steel tubular chamber about seven inches long,made of one inch pipe through which argon gas flowed at the rate ofcubic feet per hour. The tubular chamber was, in turn, located in amufile furnace which was set at 1800 F. When the specimen was beingtransferred to or from the tubular chamber, argon flow was momentarilyincreased to cubic feet per hour.

After heating at 1800 F. for ten minutes, the specimen was removed fromthe furnace and immediately transferred to another tubular chamber whichwas the duplicate of the first, but which was located outside the mufilefurnace, with the air exposure being 6 to 8 seconds. Argon also flowedthrough this chamber at a rate of 10 cubic feet per hour. After coolingfor two minutes the specimen was removed.

After this treatment the specimen had an adherent red oxide coating ofpleasing appearance. The same procedure was repeated with nine otherspecimens and the same results were obtained.

A specimen was submitted for electron diffraction analysis of the redcoating. It was identified as cuprous oxide.

To determine environmental effects on the coating, three specimens werewrapped in aluminum foil and stored for one year, three were left in thelaboratory atmosphere for one year and three were placed on a rack on abuilding roof and allowed to weather for one year. The specimens in thelaboratory retained their pleasing red color and the weathered specimendeveloped a deep rich red color.

EXAMPLE II In order to determine the effect of temperature on the colorof the coating obtained, a series of tests were made in the same way asin Example I, except that furnace temperatures from 900 F. to 1900 F. in100 F. increments were used. The results obtained in terms of colorrelated to temperature were as follows:

A mask was made from copper foil in which the letters OLIN were cut.This was placed over a heavier piece of copper and the assembly wastreated according to the procedure outlined in Example I. This resultedin the word OLIN being formed in cuprous oxide in a pleasing red coloron the copper surface.

EXAMPLE IV Two coupons of tough pitch copper measuring 2 /8" x /2 x0.022 were cleaned with steel wool until they were bright in appearance.They were then placed in a muflle furnace containing an air atmosphereat a temperature of 1800 F.

When the specimens had reached the furnace temperature, one of thespecimens was transferred to a tubular chamber through which argonflowed at room temperature at a rate of 20 cubic feet per hour. Theother specimen was placed outside the furnace in the air and allowed tocool.

When both specimens had cooled to room temperature, they were bothcovered with a black oxide scale that readily flaked ofl.

EXAMPLE V Tests were carried out as in Example I except specimens wereallowed to be exposed to air during transfer from the heating to thecooling zone for times of 30, 45 and seconds. When cooled to roomtemperature, all specimens were covered with a black non-adherent scale.

EXAMPLE VI A test was carried out as in Example I except that thespecimen was allowed to cool in air instead of in an inert atmosphere.When the specimen had cooled to room temperature, it was coated with aloosely adherent black scale of cupric oxide.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. The method of forming a red, cuprous oxide coating on copper whichcomprises: heating a copper base alloy in an inert atmosphere at atemperature of from 1100 to 1900 F.; and cooling said heated alloy to aminimum of 200 F. in an oxygen deficient zone containing up to about0.0001% oxygen.

2. 'I he method of claim 1 wherein said alloy is cooled to ambienttemperature in an oxygen-deficient zone.

3. The method of claim 1 wherein said oxygen-deficient zone containssubstantially no oxygen.

4. The method of claim 1 wherein said copper base alloy is cooled toambient temperature in less than three minutes.

5. The method of forming a red, cuprous oxide coating on copper whichcomprises: heating a copper base alloy in an inert atmosphere at atemperature of from 1100 to 1900" F.; holding said alloy at saidtemperature for at least thre minutes; and cooling said alloy to ambienttemperature in an oxygen-deficient zone containing less than 0.0001%oxygen.

6. The method of claim 5 wherein said alloy is cooled to ambienttemperature in less than three minutes.

7. The method of forming a red, cuprous oxide coating on copper whichcomprises: heating a copper base alloy in an inert atmosphere at atemperature of from 1100 to 1900 F.; exposing said heated alloy tooxygen for from 3 to 20 seconds; and cooling said alloy in inert 5atmosphere.

8. The method of forming a red, cuprous oxide coating on copper whichcomprises: heating a copper base alloy in an inert atmosphere at atemperature of from 1100 to 1900 F.; holding said alloy at saidtemperature 10 for at least three minutes; exposing said heated alloy tooxygen for from 3 to 20 seconds; and cooling in an inert atmosphere.

9. The method of claim 7 wherein said alloy is cooled to a minimum of200 F. in less than 3 minutes.

RALPH S. KENDALL, Primary Examiner.

